A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, the substrate table is generally designed to hold substrates that have standard sizes and shapes. Typically, the smallest substrate than can be held is a circular substrate with a diameter of 75 mm (approximately 3 inches). Substrate tables are generally designed so as to have a holder tailored to standard substrates; the holder should be capable of holding the substrate immobile, and designing such holders for a standard size/type of substrate allows the design to be optimized. Various other types of substrate supporting tables—such as found in auxiliary apparatus, substrate handling components, substrate transport devices, etc are also generally designed to handle substrates of a standard size and shape.
However, there are occasions when it is desirable to process a substrate that is either not of a standard size or not of a standard shape. For example, certain substrates, such as Inp, GaAs and SiC substrates generally have a diameter of 50 mm (approximately 2 inches), which is too small for most conventional substrate holders. Also, substrates may have different shapes than the standard. Typically, substrates are fully rounded or may be rounded with a flat portion along part of the circumference. However, other shapes, such as squares, rectangles and quarters of a rounded substrate of various sizes may be desirable. Moreover, it may be desirable to process irregular shapes, each of which may be unique, for example, as a result of breakage of a larger substrate into irregular pieces. However, it very expensive to design and build a substrate table and handling components specifically tailored to such non-standard substrates. Further, a user desires to use a single lithographic apparatus to be able to expose a variety of substrates, including non-standard substrates.
To address the problem of handling non-standard substrates, it is known to provide an etched recess in a standard substrate which is held by the holder on the substrate table. The non-standard substrate is then disposed in the recess. In this way, the standard substrate acts as a “host” or carrier for the non-standard substrate. However, it has been found that due to limiting tolerances in the etching process, it is difficult to consistently etch a perfect recess. The recess may often include defects. For example, the bottom surface of the recess, which acts as the supporting surface for the non-standard substrate, is may not be flat. Further, the sidewalls of the recess may not be perpendicularly oriented with respect to the bottom surface, which may lead to alignment problems due to misplacement of the non-standard substrate. Moreover, even if the desired shape is obtained, the carrier substrate may be relatively brittle, for example a silicon wafer. Thus, placing a non-standard wafer in and out of the recess may over time result in chipping pieces off the sides of the recess, which results in contamination.